In 1919, after the student leaders of the May Fourth demonstrators were jailed, Cai resigned in protest (returning to office in September). Meanwhile, he and Xu Beihong wrote regularly for the Daily University of Peking University that dealt with broader issues than just campus politics. Xu addressed issues of Art and Art History and in 1920 a university art journal called Painting Miscellany was published.[2] After resigning again in 1922, he spent a period of withdrawal in France. Returning in 1926, he supported his fellow-provincial Chiang Kai-shek and the Kuomintang's efforts to unite the country. Along with Wu Zhihui, Li Shizeng, and Zhang Renjie, he was known as one of the "Four Elders" of the Party, and a staunch anti-communist. He was appointed president of the Control Yuan, but soon resigned.[3]

Cai was frustrated in his efforts to remodel the national system of education to resemble the French system, but in 1927, he co-founded the National College of Music, which later became the Shanghai Conservatory of Music, and in April 1928, he helped to found and became the first president of the Academia Sinica. He and a wide circle of colleagues founded the China League for Civil Rights which criticized the national government and Chiang Kai-shek for abuse of power. The situation worsened, however; the League could not attain the release from jail of Chen Duxiu, Cai's former dean at Peking University, for instance. In June 1933, the Academia Sinica's academic administrator and co-founder of the League, Yang Quan, was shot and killed in the street in front of the League's Shanghai offices. After a period of shock and reflection, Cai retired from public view in a statement denouncing the political repression of the Nanjing government.[4]

After the outbreak of the Second Sino-Japanese War in 1937, partly because of declining health, instead of accompanying the national government to Sichuan, Cai moved to Hong Kong. He lived there in seclusion until his death in March 1940 at the age of 72.[5]